(1) Field of the Invention
This invention relates to a nozzle shading method and an inkjet printing apparatus using same, and more particularly to a nozzle shading method using a scanner having a narrow dynamic range.
(2) Description of the Related Art
Conventionally, an inkjet printing apparatus is susceptible to a non-discharge in which the nozzles fail to discharge ink droplets. Therefore, a non-discharge nozzle detecting mechanism is provided for detecting the non-discharge nozzles. In Patent Document 1 (Japanese Unexamined Patent Publication No. 2008-238419), for example, prints are read by a CIS (Contact Image Sensor), and non-discharge nozzles are detected based on the images read.
With an inkjet printing apparatus, variations occur in the amount of droplets discharged from each nozzle in spite of the same tone value of image data printed. The variations in the discharge among the nozzles will produce density variations of prints (see FIG. 17). In order to eliminate the density variations, nozzle shading which is a density uniforming correction (see FIG. 18) is carried out on the nozzles. Since the nozzle shading needs a highly precise reading with a large dynamic range, prints are read with a CCD (Charge Coupled Device) scanner, and ink discharge from the nozzles are adjusted according to a variation of each nozzle.